I did a search for Russian/Soviet works in the Archive and found quite a few rarities:
Napravnik String Quartet No. 3
Ippolitov Ivanov String Quartet
Jekabs Medins Horn, Bassoon, Clarinet, Viola Concerti
Aladov Concert Fantasy for Violin and Orchestra
Juzeliunas Heroic Poem
Klova Piano Concerto No. 2
Salmanov Symphonic Suite "Poetic Pictures"
Tamberg Festive Music
A. Kalnins In Memoriam
K. Khachaturian "At the Circus" Suite for Orchestra
Thank you SO much for these! And please please do let us know of many more finds!